Many types of electrical and electronic devices generate radiated and conductive interference signals. Radiated interference signals are typically broadcast through space. Conductive interference signals are typically conducted over power supply mains or power conductors. Radiated interference signals, if substantial enough in magnitude, may pose a health hazard. More typically however, conductive interference signals may adversely interfere with the proper operation of other electronic circuits which are located in close proximity and connected to the power supply mains. For these and other reasons, electronic devices are subject to governmental and regulatory restrictions limiting the amount of interference which can be emitted from such products. CISPR reports detail many of the specifics found in these regulations. The acronym "CISPR" refers to the Comite International Special des Perturbations Radioelectriques, also known as the International Special Committee on Radio Interference. The CISPR is the international committee that promotes unification by recommending approved standards to National Committees for adoption. Typically, the National Committees adopt the CISPR recommendations as their national rules in so far as national conditions will permit. Thus, regulations that set limits for interference characteristics of electrical lighting and similar equipment may be found in the CISPR reports.
Electrical switches are major sources of conductive interference in electrical control systems. Typical electrical switches include silicon controlled rectifiers (SCRs), thyristors and triacs, all of which are hereinafter generically referred to as thyristors. Generally, thyristors are three-terminal, gate-controlled, bistable AC switching devices that are incorporated in many electrical control applications. The thyristor switches the applied current on and off to regulate the delivered output power. In an AC environment, the electrical switch may switch the applied current on and off during each half cycle of AC power to regulate the power delivered to the load and therefore the output of the load. For example, the electrical control system may be a dimmer circuit which uses a thyristor to control the intensity of a light. An example of a thyristor-based dimmer circuit is described in U.S. Pat. No. Re. 35,220 which is assigned to the assignee hereof.
A thyristor generates interference signals as a result of an essentially instantaneous and virtually discontinuous current transition when switching from an off or non-conductive condition to an on or conductive condition, when a significant voltage exists across the thyristor at the time that the switching occurs. The instantaneous and discontinuous current transition is an inherent result of the switching action of the thyristor. The magnitude of the interference signal depends directly upon the magnitude of the current change rate with respect to time (di/dt). A relatively low di/dt value associated with the transition creates relatively low levels of interference. By comparison, larger di/dt values produce larger levels of interference signals.
Attempts to control the interference signals generated by thyristor-based circuits have involved the addition of auxiliary attenuating circuit elements. The attenuating circuit elements have taken the form of filters which may be as simple as a capacitor or inductor, or as complex as an elaborate multi-pole, multi-component, complex filter using both passive and active elements. U.S. Pat. Nos. 5,264,761; 5,504,394 and 5,504,395, all assigned to the assignee hereof, describe examples of such filtering and attenuation devices used with a thyristor. These auxiliary attenuating elements add to the complexity and the manufacturing expense associated with the products in which the thyristors are employed.
The other known method of reducing interference involves replacing the thyristor with a transistor-based switching circuit which provides a slower and smoother switching transition. The transistor-based switching circuit can be slowly turned on since the conduction characteristics of a power control transistor are controlled by a bias signal applied to the transistor. Slowly increasing the bias signal using simple circuit elements causes the transistor to slowly switch into full conduction. Moreover, during the transition period, current is conducted through the transistor in an amount that is proportional to the bias signal. Thus, the current change rate with respect to time (di/dt) through the transistor can be lowered based upon the simple circuit elements used to produce a gradual change in the bias signal. Since the magnitude of the interference signals is directly related to the magnitude of the di/dt, decreasing the di/dt reduces the magnitude of any generated interference signals. Typically, because of the lower di/dt, the transistor-based switching circuit avoids generating significant interference signals and is thus capable of complying with the pertinent regulations.
Although the transistor-based circuits provide the desirable slow turn-on characteristics and thus attenuate interference generation, they consume relatively high amounts of power. Usually the power consumption of a fully conductive transistor-based circuit is much higher than that of a thyristor. The forward conduction voltage of the power transistor and the forward bias voltages of the additional circuit elements result in significant power consumption. For example, since the power transistors are direct current devices, the power transistor requires a diode bridge to rectify the AC current. Since the power consumption for the circuit directly relates to the sum of the voltages across all circuit elements, the additional diode bridge voltages substantially increase the overall circuit power consumption.
The increased power consumption translates into more heat generated by the transistor-based switching circuit. Since high levels of heat will destroy the semiconductor elements, heat sinks are usually required to dissipate the heat. Heat sinks are relatively large and the addition of heat sinks to the circuit increases its overall size. The resulting size may be too large to integrate such a transistor-based switching circuit into small spaces such as lamp sockets and lamp bases. Furthermore, the useful longevity of the semi-conductor elements is decreased in a high-heat environment.
The thyristor-based circuits, on the other hand, do not consume large amounts of power nor do they generate significant amounts of heat. No rectification diodes are necessary. Therefore, when in conduction, the thyristor is the primary circuit element conducting current and consuming power, and the power consumed by the circuit is dependent primarily on the forward voltage characteristics of the thyristor. The typical power consumption is substantially less than the typical power consumption of a transistor-based switching circuit. Unfortunately, however, the large di/dt values and unacceptable interference problems associated with known thyristor-based switching circuits may restrict the use of thyristors, despite low power consumption.
It is with respect to these and other factors that the present invention has evolved.